Previous Next

                                                509
     SECTION 6.5                                                    Scan Conversion Details



     Note: Although the figure exaggerates the difference between the curved and flat-
     tened paths for the sake of clarity, the purpose of the flatness tolerance is to control
     the precision of curve rendering, not to draw inscribed polygons. If the parameter’s
     value is large enough to cause visible straight line segments to appear, the result is
     unpredictable.




                                        Flatness error
                                          tolerance




                                  FIGURE 6.6 Flatness tolerance


6.5.2 Smoothness Tolerance

     The smoothness tolerance (PDF 1.3) controls the quality of smooth shading
     (type 2 patterns and the sh operator) and thus indirectly controls the rendering
     performance. Smoothness is the allowable color error between a shading approx-
     imated by piecewise linear interpolation and the true value of a (possibly non-
     linear) shading function. The error is measured for each color component, and
     the maximum error is used. The allowable error (or tolerance) is expressed as a
     fraction of the range of the color component, from 0.0 to 1.0. Thus, a smoothness
     tolerance of 0.1 represents a tolerance of 10 percent in each color component.
     Smoothness can be specified as the value of the SM entry in a graphics state
     parameter dictionary (see Table 4.8 on page 220).

     Each output device may have internal limits on the maximum and minimum
     tolerances attainable. For example, setting smoothness to 1.0 may result in an in-
     ternal smoothness of 0.5 on a high-quality color device, while setting it to 0.0 on
     the same device may result in an internal smoothness of 0.01 if an error of that
     magnitude is imperceptible on the device.

Previous Next